This invention concerns a device for the production of metal iron by means of the direct reduction of mineral iron, wherein the iron is present in the form of oxides, by means of a direct reduction of said oxides
The device according to the invention comprises a reactor which is at least partly shaped like a truncated cone and wherein the various processes take place which achieve the direct reduction of the iron oxides.
The reduced iron can emerge from the reactor either hot or cold and subsequently can be sent to a melting furnace to produce liquid steel, or can be converted into hot brick iron (HBI), or again it can be transported into a cooling and storage zone
In correspondence with one or more different longitudinal zones, the reactor is provided with a conduit equipped with nozzles through which reducing gas is injected.
This invention is characterized by the fact that the reduction reactor has a multiple taper conformation, diverging by at least an angle in its upper part and converging by at least an angle in its lower part.
In the field of steel production, the use of reduced iron (DRI) as a loading material for melting or conversion processes is more and more common.
The process to obtain reduced iron provides to make the mineral iron react with a current of reducing gas in an appropriate device comprising a reaction container, called the reactor, defining in its height at least a zone wherein the reduction process occurs.
The devices used are generally of the gravitational type, also called shaft types, and comprise a central part, with a substantially cylindrical or truncated cone shape, a cylindrical upper zone for loading, a lower zone for discharge, means to inject reducing gas into one or more zones of the reactor and means to create an intake of the gases, at least in the upper zone.
In order to optimize the performance of the chemical processes to reduce the iron oxides, it is necessary to create conditions of uniform distribution, inside the reduction device, both of the load of mineral introduced and also of the reducing gas.
In conventional reactors, particularly large size ones, the flow of reducing gas introduced laterally prevalently affects the peripheral zone: this gives a reduced yield of the reduction reactions in correspondence with the central zone.
Moreover, in traditional reactors blockages of material are often created in the upper part, particularly with certain types of material, and/or the material sticks on the walls when the material to be reduced comes into a partly plastic state
Furthermore, if the injection of the current of gas occurs in a reduction zone of the reactor where the diameter is too large, this entails poor efficiency and therefore low yield of the reduction process.
Irregularities in the flow of material and gas inside the reactor cause a poor yield in the reduction process, and negatively affect the productivity of the device.
The lower part of the reactor, converging downwards, conventionally has a constant taper.
With this conformation, the volume of material passing through is very limited and, to keep productivity high, the time the solid material remains inside the reactor is also limited.
Therefore, the carbon (C) is not given the necessary time to spread efficaciously in the molecular structure of the metal, and therefore it is not possible to obtain the desired compounds of Fe and C, such as for example Fe3C. DE-C-198 38 368 discloses a reactor for the direct reduction of iron material which comprises, in its upper part, a tubular inner prevacuum chamber able to uniformly spread the charge of material introduced into the reactor from the above.
This chamber has also the function of dividing, in the upper part of the reactor, the central inner zone, through which the charge of iron material is fed into the reactor, from the peripherical annular zone which is empty and through which the gasses exiting from the inner of the reactor are made to transit.
This chamber has no function of pre-heating or reducing of the iron oxides fed into the reactor
The present Applicant has devised and embodied this invention to overcome all these shortcomings, to improve the efficiency of the process and the quality of the product
The reduction device according to the invention is of the gravitational or shaft type, wherein both the material and the gas are advantageously fed continuously, so as to create a vertical and gravitational flow of the material and to achieve the direct reduction of the mineral.
The reduction device according to the invention is equipped with means to feed the mineral iron and means to discharge the reduced metal iron.
The device is also equipped with conduits to inject the reducing gas in correspondence with one or more zones distributed on the height of the reactor.
One purpose of the invention is to achieve a reduction device in which there is a stable and uniform distribution both of the load of metal and also of the reducing gas throughout the volume full of mineral iron, so as to obtain high productivity, a better quality of the reduced iron and a greater quantity of carbon, possibly as Fe3C.
Another purpose of the invention is to achieve a device wherein the load material is prevented from amassing and blocking in correspondence with the upper part of the reactor, and which avoids the risks of the superheated material sticking against the walls of the reactor.
A further purpose of the invention is to encourage and facilitate the descent of the reduced material, in the lower part of the reactor, towards the outlet from the reactor, at the same time improving the efficiency of the injection of the gas in said zone and increasing the volume available for reaction.
According to the invention, the reduction device comprises a reactor defined by a first upper zone, with a taper diverging downwards, and a second lower zone, with a taper converging downwards.
According to a preferential embodiment of the invention, the second lower zone is defined by at least two segments equipped with respective angles of convergence which are different from each other.
The first upper zone defines a heating, pre-reduction and final reduction zone where, thanks to the introduction of currents of reducing gas into at least one circumferential zone, the following transformation reactions are achieved: Fe2O3xe2x80x94 greater than Fe3O4, Fe3O4xe2x80x94 greater than FeO and FeOxe2x80x94 greater than Fe.
The second lower zone comprises the transition zone and the zone where the metallized material is carburized and cooled.
According to a variant, between the divergent upper zone and the convergent lower zone there is a substantially cylindrical separation segment wherein the reduction reactions are completed.
The divergent conformation of the first upper zone encourages a better distribution of the load inside the reactor and a better distribution of the gas over the whole inner volume.
The better distribution of the load and gas causes a higher heat yield of the chemical reactions, which can take place faster and with a consequent increase in productivity. in the higher part of the reactor, the downwardly divergent form encourages the downward flow of the material, preventing it from sticking to the walls.
During the reaction of the Fe2O3 to Fe3O4, the mineral iron increases in volume by a value which can vary from 15 to 30%, according to the conditions of the process and the type of material loaded.
This increase in volume causes a corresponding increase in the pressure on the pellets of material introduced, thus increasing the risk of their sticking to the walls.
The divergent conformation of the reactor in its upper part increases the volume available as the material descends, preventing blockages and allowing the volume to increase freely.
In the peripheral zones, moreover, there is no longer any pressure exerted by the column of material, which reduces the chances of sticking.
According to the invention, the angle of aperture of the first divergent upper part of the reactor with respect to the vertical is between 1 and 5 degrees, advantageously around about 3 degrees.
The first upper part has an extension in height, according to the invention, of between about xc2xcand about xc2xdof the overall height of the reactor.
According to another embodiment, the first upper part has a conformation defined by two or more consecutive segments having a different angle of divergence to the vertical
The convergent conformation of the second lower part causes an increase in the efficiency of injection of the gas, due to the reduction in the diameter of the section of the reactor where the gas is introduced.
In the lower part of the reactor, the downwardly converging form encourages a decrease in the speed of the gas as the gas gradually rises from the bottom upwards.
In this way, the time available for the gas to complete the reactions increases, with regard to carburization, so that carbon is obtained in the form of Fe3C; there is also more time for the gas to exchange heat with the material, thus allowing the gas to cool.
According to a preferential embodiment, the taper of the lower part of the reactor has two or more segments with a progressively larger taper.
This embodiment allows to adapt the form of the terminal segment of the reactor as the temperature of the material varies.
In fact, as it gradually descends inside the reactor, the material cools and thus its tendency to stick to the walls decreases.
Thus the volume available in the lower zone of the reactor is increased and the conditions for carburization and cooling are optimized.
Moreover, the reduced material is unloaded more quickly and efficiently towards the outlet zone and the discharge means.
According to the invention, the angles of convergence of the second lower zone are between 5 and 20 degrees, advantageously between 8 and 15 degrees, to the vertical.